It is now common practice to explore the oceans of the earth for deposits of oil, gas and other valuable minerals by seismic techniques in which an exploration vessel imparts an acoustic wave into the water, typically by use of a compressed air "gun". The acoustic wave travels downwardly into the sea bed and is reflected at the interfaces between layers of materials having varying acoustic impedances. The wave travels back upwardly where it is detected by microphone or "hydrophone" elements towed in the water by the vessel to yield information regarding characteristics of the underwater material and structures.
A towed acoustic streamer comprises a plurality of pressure-sensitive hydrophone elements enclosed within a waterproof sheath, given a near-neutral buoyancy and electrically coupled to recording equipment on board the vessel. Streamers that are suspended vertically rather than being towed have no need for the waterproof sheath because they need not be neutrally buoyant. Often, multiple streamers are towed in parallel to form a two-dimensional array of hydrophones. For purposes of this discussion, however, "streamer" and "array" are used somewhat interchangeably.
Each hydrophone element within the towed array is designed to convert the mechanical energy present in pressure variations surrounding the hydrophone element into electrical signals. Most typically, this is done by constructing the hydrophone of a piezoelectric material, such as lead zirconate titanate ("PZT") and a means by which to amplify pressure variations to obtain the strongest possible signal (often by one or more diaphragms acting as tympanic collectors). The hydrophone elements are typically provided with leads or contacts to which to join electrical conductors, the electrical conductors carrying signals from the hydrophone elements to the recording equipment.
A typical towed array is taught in U.S. Pat. No. 4,160,229, which issued on Jul. 3, 1979, directed to a hydrophone streamer apparatus embodying concentric tube construction for achieving improved low noise operation. A plurality of hydrophone elements are supported within a compliant inner tube at spaced intervals therealong by rather complicated compliant mounting means. The inner tube is supported within an elongated outer jacket by compliant support means between the outer surface of the inner tube and the inner surface of the jacket. Suitable support means may comprise a plurality of trilobate devices each formed of three tubular sections equally spaced around the inner tube, the trilobate devices being located along the inner tube at positions between adjacent transducer elements.
During operation, the towed array is surrounded by water. Because a piezoelectric hydrophone element is a high impedance device, any salt water coming into contact with the element causes leakage paths for electrical current present in the leads thereto, either severely distorting the signal produced by the hydrophone element or shorting the hydrophone element entirely. Therefore, it is very important to keep the hydrophone element as free of saltwater contact as possible. Normally, hydrophone elements are immersed in a hydrocarbon fill fluid to give the streamer near neutral buoyancy and to insulate the element from saltwater contact to a certain extent. However, accidental intrusion of saltwater into a streamer is not infrequent. In the past, substantial effort has been directed to solving the problem of waterproofing hydrophone elements when saltwater intrusion of the streamer occurs.
For instance, U.S. Pat. No. 3,258,739, which issued on Jun. 28, 1966, is directed to a piezoelectric hydrophone featuring, among other things, a water resistant skin to keep the hydrophone from contacting the surrounding water.
U.S. Pat. No. 4,782,470, which issued on Nov. 1, 1988, is directed to a pressure-sensitive element having an acoustically transparent water impervious layer interposed and bonded between an exterior surface of the pressure-sensitive element and an interior surface of a rubber housing.
U.S. Pat. No. 3,418,624 is directed to a coaxially mounted line hydrophone having a hollow, substantially cylindrical piezoelectric hydrophone element therein. Electrical leads to and from the element pass through a center of the element. The element is encased in a sound transparent, waterproof jacket of material, such as rubber.
Unfortunately, the structures described above are either difficult or expensive to manufacture or degrade over time due to mechanical flexing of the waterproof coating or chemical interaction of the coating with the surrounding fluid. The above structures also subject the pressure wave to undesirable levels of distortion prior to its contact with the hydrophone element.
At one time, the analog signals output by the hydrophones were typically transmitted to the vessel where they were digitized and stored for later processing and analysis. Since the analog signals were in a frequency range of generally some tens of Hertz, electromagnetic interference with the hydrophone element or the electrical leads extending therefrom were often less of a problem. Today, however, analog signals from hydrophones are typically digitized within the streamer itself and transmitted at high frequencies. Therefore, in the interest of optimizing signal quality in light of these higher frequencies, it has become imperative to provide effective shielding as against stray electromagnetic fields.
It should be recognized first that the output from the hydrophones is in the form of a very faint signal and that this low signal level is obviously very susceptible to noise. Noise in the marine exploration environment can come from many sources and takes many forms. Mechanical noise is generated by the towing ship, by other shipping in the vicinity and by vibration of the streamer both along its length and side-to-side with respect to the direction of travel. The turbulence generated by towing this streamer through the water is also a source of substantial noise. Electrical noise is generated by electronic components and wires proximate the hydrophone inducing electrical currents in either the hydrophone element or its leads. Noise, in the sense of distortion of the signal, is also generated by inaccuracy in the digitization processes.
British published patent application no. 2485226 A, which was published on Mar. 20, 1985, is directed to solution of interference from mechanical noise and teaches a streamer for towing behind a marine vessel engaged in seismic exploration which comprises a plurality of substantially identical modules connected serially to one another, each module comprising plural hydrophones for detecting reflected acoustical waves and producing analog electronic signals in response thereto, a digitizer for converting the analog signals to digital representations thereof and a multiplexed transmitter for sending the digital representations along a conductor within the modules up the streamer to the vessel for recording. Each module comprises a polymer or elastomer wall, open cell foam, a fill fluid of an appropriate specific gravity and Kevlar strength members. Noise caused by digitization can be reduced by the techniques described in British published patent application nos. 2130829 and 2131241.
Isolation from electromagnetic fields is a different proposition. U.S. Pat. No. 4,819,216, which issued on Apr. 4, 1989, is directed to a towed array configuration having an integrated hydrophone, preamplifier and telemetry hybrid unit suitable for connecting with a single coaxial cable. The preamplifier and FM hydrophone telemetry assembly comprises an air-backed ceramic assembly connected to a preamplifier/voltage control oscillator chip. In addition, the chip includes a regulator and power separation network. The configuration is enclosable within an electrostatic shield to provide partial isolation from electromagnetic fields.
U.S. Pat. No. 4,733,375, which issued on Mar. 22, 1988, is directed to a flexible line array transducer assembly for detecting underwater acoustical signals. The assembly includes an array of spaced-apart piezoelectric elements arranged generally in a line and selected to have low cross-coupling characteristics, low sensitivity to incoherent mechanical perturbations in the directions longitudinal and lateral to the axis of the array and high sensitivity to coherent mechanical perturbations, such as acoustical signals. The elements are polarized in a direction generally transverse to the array and each include opposing surface areas which are generally parallel with the linear axis of the array. Electrodes are disposed on the opposing surface areas of the elements and are coupled to conductors which carry signals produced by the piezoelectric elements when the elements are stressed by acoustical signals. A porous, open-cell material is disposed about the piezoelectric elements as a non-waterproof encasement to maintain the elements in place and mechanically isolate the elements. An outer, water-tight jacket encloses the open cell material and holds a fill fluid carried within the open-cell material. An electrically conductive flexible sleeve may be placed either about the open-cell material or about the outer jacket to shield the piezoelectric elements from electromagnetic waves.
Unfortunately, the above-described devices are complicated and expensive to manufacture and may not hold up over years of use. They are also vulnerable to water infusion, causing leakage paths about the hydrophone elements.
As mentioned before, a typical piezoelectric hydrophone element consists of at least one diaphragm. As with all mechanical systems, the hydrophone element has modes of vibration wherein the element generates false signal levels in response to vibration that is sympathetic to external vibration in one or more of these modes. None of the devices described above had attenuation of these sympathetic modes as a specific design objective, although they did attenuate modes to a limited extent by coincidence.
What is needed in the art is a relatively inexpensive scheme for providing (1) water- or fluid-proofing, (2) shielding from stray electromagnetic fields and (3) damping of sympathetic vibrational modes for hydrophone elements.